materials Article Carbon Adsorbents from Spent Coffee for Removal of Methylene Blue and Methyl Orange from Water Inga Block 1 , Christina Günter 2 , Alysson Duarte Rodrigues 3 , Silvia Paasch 4 , Peter Hesemann 3 and Andreas Taubert 1, *   Citation: Block, I.; Günter, C.; Duarte Rodrigues, A.; Paasch, S.; Hesemann, P.; Taubert, A. Carbon Adsorbents from Spent Coffee for Removal of Methylene Blue and Methyl Orange from Water. Materials 2021, 14, 3996. https://doi.org/10.3390/ma14143996 Academic Editor: Zhenghong Huang Received: 20 May 2021 Accepted: 1 July 2021 Published: 16 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Institut für Chemie, Universität Potsdam, Karl-Liebknecht-Straße 24-25, D-14476 Potsdam, Germany; iblock@uni-potsdam.de 2 Institut für Geowissenschaften, Universität Potsdam, Karl-Liebknecht-Straße 24-25, D-14476 Potsdam, Germany; guenter@geo.uni-potsdam.de 3 ICGM, Univ Montpellier-CNRS-ENSCM, 34090 Montpellier, France; alysson.duarte-rodrigues@umontpellier.fr (A.D.R.); peter.hesemann@umontpellier.fr (P.H.) 4 Professur für Bioanalytische Chemie, TU Dresden, Bergstraße 66, D-01062 Dresden, Germany; silvia.paasch@tu-dresden.de * Correspondence: ataubert@uni-potsdam.de; Tel.: +49-(0)3-3197-75773 Abstract: Activated carbons (ACs) were prepared from dried spent coffee (SCD), a biological waste product, to produce adsorbents for methylene blue (MB) and methyl orange (MO) from aqueous solution. Pre-pyrolysis activation of SCD was achieved via treatment of the SCD with aqueous sodium hydroxide solutions at 90 ◦ C. Pyrolysis of the pretreated SCD at 500 ◦ C for 1 h produced powders with typical characteristics of AC suitable and effective for dye adsorption. As an alternative to the rather harsh base treatment, calcium carbonate powder, a very common and abundant resource, was also studied as an activator. Mixtures of SCD and CaCO 3 (1:1 w/w) yielded effective ACs for MO and MB removal upon pyrolysis needing only small amounts of AC to clear the solutions. A selectivity of the adsorption process toward anionic (MO) or cationic (MB) dyes was not observed. Keywords: water; spent coffee; dye adsorption; methylene blue; methyl orange; calcium carbonate; activated carbon; water treatment; dye removal 1. Introduction Water is among the most valuable resources worldwide. As a result, reliable methods for water treatment and remediation are among the most pressing issues worldwide. In addition to natural organic matter, heavy metals, biological contaminants, and disinfection byproducts are among the key pollutants in surface and drinking water. Synthetic organic substances such as dyes, oils, or pharmaceuticals and their metabolites or degradation products also play a major role in water pollution [1]. Many of these substances can be removed from the aqueous phase by activated carbon (AC) adsorbents. AC can be produced on an industrial scale from sources such as coal, petroleum, or coconut husk [2,3]. As a general requirement, the starting material needs to have a high carbon content, and many renewable raw materials are therefore candidates for AC production. Typical ACs have large surface areas between 500 and 1500 m 2 /g and pore volumes of up to 1.8 cm 3 /g [1]. Therefore, many groups have studied a diverse pool of natural (waste) products such as coconut shells, fruit seeds (grapes, pomegranate, date palm), olive and peach stones, wood wastes, pine cones, and more for the fabrication of ACs [4–20]. Other studies include composites of clay and carbonaceous materials [6,21–23]. For example, Okman and Karagöz et al. [10] chemically activated grape seeds with potassium carbonate and potassium hydroxide before carbonization between 600 and 800 ◦ C, resulting in materials with BET surface areas of up to 1238 m 2 /g (K 2 CO 3 ) and 1222 m 2 /g (KOH). Materials 2021, 14, 3996. https://doi.org/10.3390/ma14143996 https://www.mdpi.com/journal/materials